A high percentage of hospitals and similar institutions utilize ultrasonic diagnostic equipment. There is a substantial need by such institutions, as well as by manufacturers of such ultrasonic equipment and by research organizations in this field, for a compact and portable apparatus for measuring the radiated power output of diagnostic and other ultrasonic equipment in a reliable and safe manner. Prior to the development of the device of the present invention there were no conveniently portable devices available capable of measuring the total time-averaged power output of diagnostic ultrasound units. Laboratory thermal measurement devices were available, but these were not of the parabolic focussing type and therefore had low sensitivity and relatively slow response.